Photovoltaic Device Including A Back Contact And Method Of Manufacturing

ABSTRACT

A photovoltaic device includes a substrate, a transparent conductive oxide, an n-type window layer, a p-type absorber layer and an electron reflector layer. The electron reflector layer may include zinc telluride doped with copper telluride, zinc telluride alloyed with copper telluride, or a bilayer of multiple layers containing zinc, copper, cadmium and tellurium in various compositions. A process for manufacturing a photovoltaic device includes forming a layer over a substrate by at least one of sputtering, evaporation deposition, CVD, chemical bath deposition process, and vapor transport deposition process. The process includes forming an electron reflector layer over a p-type absorber layer.

RELATED APPLICATIONS

This application claims priority from U.S. provisional application Ser.No. 61/804,469, filed on Mar. 22, 2013, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

Disclosed embodiments relate generally to photovoltaic devices and, inparticular, to photovoltaic devices having a back contact layer.

BACKGROUND OF THE INVENTION

A photovoltaic device generates electrical power by converting lightinto direct current electricity using semiconductor materials thatexhibit the photovoltaic effect. The photovoltaic effect generateselectrical power upon exposure to light as photons, packets of energy,are absorbed within the semiconductor to excite electrons to a higherenergy state. These excited electrons are thus able to conduct and movefreely within the material

A basic unit of photovoltaic device structure, commonly called a cell,may generate only small scale electrical power. Thus, multiple cells maybe electrically connected to aggregate the total power generated amongthe multiple cells within a larger integrated device, called a module orpanel. A photovoltaic module may further comprise a protective backlayer and encapsulant materials to protect the included cells fromenvironmental factors. Multiple photovoltaic modules or panels can beassembled together to create a photovoltaic system, or array, capable ofgenerating significant electrical power up to levels comparable to othertypes of utility-scale power plants. In addition to photovoltaicmodules, a utility-scale array would further include mountingstructures, electrical equipment including inverters, transformers, andother control systems. Considering various levels of device, fromindividual cell to utility-scale arrays containing a multitude ofmodules, all such implementations of the photovoltaic effect may containone or more photovoltaic devices to accomplish the energy conversion.

Thin film photovoltaic devices are typically made of various layers ofdifferent materials, each serving a different function, formed on asubstrate. A thin film photovoltaic device would include a frontelectrode and a back electrode to provide electrical access to thephotoactive semiconductor layer or layers sandwiched between.

In one example of a photovoltaic device, the substrate would be a glasssheet, such as soda lime glass, float glass, or low iron glass, butcould also be a polymer or other suitable material. The substrate mayhave various surface coatings on the internal and external surfaces, inthe context of the finished device. That is, the external surface isexposed to the environment, and the internal surface is encapsulatedwithin the photovoltaic device. The surface coating on the externalsurface may include an anti-reflective coating, an anti-soiling coatingor other coating to improve the device performance. The substrate mayinclude coatings on the internal surface, such as a buffer layer,transparent conductive oxide (TCO) layer and a barrier layer.

The internal coatings together comprise a TCO stack. The barrier layerlessens diffusion of sodium or other contaminants from the substrate tothe semiconductor layers. The buffer layer decreases the likelihood ofirregularities occurring during the formation of the semiconductor layeror layers. The TCO layer is a transparent, electrically conductive,material serving as a front electrode to the photovoltaic device tocommunicate a generated electrical current to a circuit, which mayinclude an adjacent photovoltaic device, such as to adjacent cellswithin a photovoltaic module.

The semiconductor layer or layers will typically include a p-n junctionthat drives an electrical current as light is absorbed within thematerial. A p-n junction may be formed by of a bilayer where the firstlayer is an n-type layer referred to as the window layer and where thesecond layer is a p-type layer referred to as the absorber layer. Whenlight is incident on the photovoltaic device, photons will exciteelectrons to a higher energy level causing them to conduct within thematerial. Front and back electrodes are connected to the semiconductorlayer or layers to provide a front and back current pathways to takeadvantage of the conducting electrons. The efficient operation of thedevice, that is, how much light energy incident on the device isconverted and collected as usable electrical power, may be negativelyaffected by losses as the generated current flows between adjacentlayers of dissimilar materials. These losses may include resistancelosses, and may also include loses due to recombination of mobile chargecarriers.

The manufacturing of a photovoltaic structure generally includessequentially forming the functional layers through a process that mayinclude vapor transport deposition, atomic layer deposition, chemicalbath deposition, sputtering, closed space sublimation, or any othersuitable process that creates the desired material. Once a layer isformed it may be desirable to modify the physical characteristics of thelayer through subsequent treatments processes. For example, a treatmentprocess step may include passivation, which is defect repair of thecrystalline grain structure, and may further include annealingImperfections or defects in the crystalline grain of the materialdisrupt the periodic structure in the layer and can create areas of highresistance or undesirable current pathways, for example, parallel to butseparated from the desired current pathway such as a shunt path orshort.

An activation process may accomplish passivation through theintroduction of a chemical dopant to the semiconductor bi-layer as abathing solution, spray, or vapor. Subsequently annealing the layer inthe presence of the chemical dopant at an elevated temperature providesgrain growth and incorporation of the dopant into the layer. The largergrain size reduces the resistivity of the layer, allowing the chargecarriers to flow more efficiently. The incorporation of a chemicaldopant may also make the regions of the bi-layer more n-type or morep-type and able to generate higher quantities of mobile charge carriers.Each of these improves efficiency by increasing the maximum voltage thedevice can produce and reducing unwanted electrically-conductiveregions. In the activation process, the parameters of annealtemperature, chemical bath composition, and soak time, for a particularlayer depend on that layer's material.

Therefore, it is desirable to provide an effective back electricalconnection at the back of the photovoltaic device to minimize lossesthat may occur at the interface between the absorber layer and the backcurrent pathway and a method of making such a photovoltaic device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic of functional layers in a first embodiment ofa photovoltaic device.

FIG. 2 depicts a schematic of functional layers in a second embodimentof a photovoltaic device.

FIG. 3 depicts a schematic of functional layers in a third embodiment ofa photovoltaic device.

FIG. 4 depicts a process for manufacturing a photovoltaic device.

FIG. 5 depicts an expanded step of a process for manufacturing a layerin a photovoltaic device.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following detailed description and appended drawings describe andillustrate various exemplary embodiments. The description and drawingsserve to enable one skilled in the art to make and use the embodimentsand are not intended to impart any limitation to the scope of theembodiments. In respect of the methods disclosed, the steps presentedare exemplary in nature and, thus, the order of the steps is notnecessary or critical.

Each of the layers described in the following embodiments may becomposed of more than one layer or film. Additionally, each layer cancover all or a portion of the device and/or all or a portion of thelayer or material underlying the layer. For example, a “layer” can meanany amount of material that contacts all or a portion of a surface.During a process to form one of the layers, the created layer forms onan outer surface, typically a top surface, of a substrate or substratestructure. A substrate structure may include a base layer introducedinto a deposition process and any other or additional layers that mayhave been deposited onto the base layer in a prior deposition process orprocesses. Layers may be deposited over the entirety of a substrate withcertain portions of the material later removed through laser ablation,scribing, or other material-removal process.

Several specific embodiments of a novel photovoltaic device will bedescribed with reference to the figures. A novel photovoltaic deviceaccording to the disclosed embodiments can include electron reflectorlayers providing an ohmic contact to achieve high performance efficiencyat the interface between the absorber layer and the back currentpathway. In a first embodiment, an electron reflector layer includingzinc telluride (ZnTe) doped with copper telluride (Cu₂Te) is disclosed.In a second embodiment, an electron reflector layer including aZnTe/Cu₂Te alloy is disclosed. In a third embodiment, an electronreflector layer is disclosed including a bi-layer of materials havingdifferent compositions, with a first layer including ZnTe or cadmiumzinc telluride (CZT) and a second layer including Cu₂Te in multipleparticular combinations.

To improve the flow of electrical current in a photovoltaic device, atechnique called electron reflection may be used. An electron reflectorreduces charge loss between the absorber layer and the back currentpathway by reducing the recombination of electron-hole pairs at thesurface of the absorber layer closest to the back current pathway.Electron reflection is practically applied, for example, by the additionof an electron reflector material layer between the absorber layer andthe back current pathway. The electron reflector layer is capable ofproviding a conduction-band energy barrier at the junction between theabsorber layer and the metal back conductor.

By way of example, an electron reflector material layer between acadmium telluride (CdTe) absorber layer and a back current pathway ispreferably formed from a material which has a higher band gap thancadmium telluride (CdTe) wherein the band gap is the energy required toexcite valence electrons to the conduction band of the material whereelectron movement as electrical current can occur. The electronreflector layer thus provides a conduction-band energy barrier whichrequires higher energies for electron movement, reducing the number ofelectrons having the tendency and energy to migrate to the surface ofthe absorber layer and into or across the electron reflector layer.

The performance of the photovoltaic device can be further improved wherethe electron reflector layer also provides a low-resistance ohmiccontact at the interface between the electron reflector layer and theabsorber layer. Low resistance ohmic contacts can limit the powerdissipation via heat generation.

A photovoltaic device 100 according to the present disclosure isdepicted in FIG. 1. Making up the photovoltaic device 100, an n-typewindow layer 115 comprising, for example, cadmium sulfide (CdS), isdeposited over a substrate structure including a base layer 105 and aTCO layer 110. A p-type absorber layer 120 comprising, for example,cadmium telluride (CdTe), is deposited over the window layer 115. Thewindow layer 115 and the absorber layer 120 form a p-n junction. Anelectron reflector layer 130 is deposited over the absorber layer. Aback electrode layer 140 and a back contact layer 150 are formed overthe electron reflector layer 130. Together, the back electrode layer 140and the back contact layer comprise the back current pathway for thephotovoltaic device as depicted.

The base layer 105 may include glass, for example, soda lime glass,float glass or low-iron glass. Alternatively, the base layer 105 mayinclude polymeric, ceramic, or other materials that provide a suitablestructure for forming a base of photovoltaic cell. Preferably, the baselayer 105 transmits light through its thickness with minimal or noabsorption or reflection of photons. The base layer 105 provides asubstrate surface upon which further layers of material are sequentiallyformed to create the photovoltaic device. The base layer 105 may haveadditional layers applied (not shown) that improve the transmission ofphotons through its thickness, which may include anti-reflectivecoatings or anti-soiling coatings.

The base layer 105 optionally may have additional layers applied (notshown) that promote the chemical stability of the glass, which mayinclude a barrier layer that inhibits the diffusion of chemical ionsfrom, into, or across the glass substrate. The barrier layer may beformed of, for example, silicon nitride, silicon oxide, aluminum-dopedsilicon oxide, boron-doped silicon nitride, phosphorus-doped siliconnitride, silicon oxide-nitride, or combinations or alloys thereof. Thebarrier layer can be formed over the base layer 105 through variousdeposition methods including chemical vapor deposition, molecular beamdeposition, sputtering, spray pyrolysis, and other conventional methods.

The TCO layer 110 allows light to pass through to a semiconductor windowlayer 115 while serving as an ohmic electrode to transportphotogenerated current away from the light-absorbing p-n junction as thefront current pathway. The TCO layer 110 may include, for example, tinoxide, zinc oxide, indium gallium oxide, cadmium stannate, cadmium tinoxide, cadmium indium oxide, fluorine doped tin oxide, aluminum dopedzinc oxide, or indium tin oxide, combinations and doped variationsthereof, or any other suitable material. The TCO layer 110 can be formedover the base layer 105, or a barrier layer (if included), throughvarious deposition methods including chemical vapor deposition,molecular beam deposition, sputtering, spray pyrolysis, and otherconventional methods.

The TCO layer 110 optionally may further include additional materiallayers applied over its surface (not shown), as a buffer layer thatpromotes the electrical function of the TCO or that provides an improvedsurface for the subsequent deposition of semiconductor materials. Thebuffer layer may be formed of, for example, tin oxide, zinc tin oxide,zinc oxide, zinc oxysulfide, or zinc magnesium oxide. The buffer layercan be formed over the TCO layer 110, through various deposition methodsincluding chemical vapor deposition, molecular beam deposition,sputtering, spray pyrolysis, and other conventional methods.

The window layer 115 may include an n-type semiconductor and forms then-type region of the p-n junction within the photovoltaic device 100.The window layer 115 preferably allows sunlight to pass through itsthickness and may be formed of a cadmium sulfide (CdS) material thatfurther includes impurities or dopants in the CdS bulk material. Thewindow layer 115 may be between 10 nm to 100 nm thick or alternativelybetween 30 nm to 75 nm thick. The window layer 115 may be formed overthe TCO layer 110, or a buffer layer disposed thereon, by a depositionprocess, such as vapor transport deposition, atomic layer deposition,chemical bath deposition, sputtering, closed space sublimation, or anyother suitable process.

The p-type absorber layer 120 may include a p-type semiconductormaterial to form the p-type region of the p-n junction within thephotovoltaic device 100. The absorber layer 120 preferably absorbsphotons passing through from the window layer 115 to mobilize chargecarriers. The absorber layer 120 may be formed of CdTe, Cadmiumsulphotelluride (Cd_((1-x))S_(x)Te, 0<x<1), or include other materialsor alloys. An absorber layer 220 formed of CdTe may further includeimpurities or dopants in the CdTe bulk material. The absorber layer 120may be between 500 nm to 8000 nm thick, or alternatively between 1000 nmto 3500 nm thick. The absorber layer 120 may be formed over the windowlayer 115 by a deposition process, such as vapor transport deposition,atomic layer deposition, chemical bath deposition, sputtering, closedspace sublimation, or any other suitable process.

The back contact layer 150 and back electrode layer 140 are providedopposite to the TCO layer 110, sandwiching the semiconductor layers ofthe photovoltaic device 200. The back contact layer 150 and backelectrode layer 140 serves as a second ohmic electrode to transportphotogenerated electrical current through the back current pathway. Theback electrode layer 140 can include electrically conductive materials,such as molybdenum nitride (MoN_(x)), chromium nitride (CrN_(x)),silver, nickel, copper, aluminum, titanium, palladium, chrome,molybdenum, gold, or combinations thereof. The back contact layer 150and can include electrically conductive materials, such as tin, silver,nickel, copper, aluminum, titanium, palladium, chrome, molybdenum, gold,or combinations thereof. The back contact layer 150 and back electrodelayer 140 can be formed through various deposition methods includingchemical vapor deposition, molecular beam deposition, sputtering, spraypyrolysis, and other conventional methods. Alternative configurationswhere the back electrode layer 140 and the back contact layer 150 are inreverse position with respect each other are also possible (not shown).

The TCO layer 110 may form or may be electrically connected to a frontcurrent pathway through which the electrical current generated by theactive layers of the photovoltaic device may flow. The back contactlayer 150 and back electrode layer 140 may form or may be electricallyconnected to a back current pathway through which the electrical currentgenerated by the active layers of the photovoltaic device may flow. Thefront current pathway may connect one photovoltaic cell to an adjacentcell in one direction within a photovoltaic module or, alternatively, toa terminal of the photovoltaic module. Likewise, the back currentpathway may connect the photovoltaic cell to a terminal of thephotovoltaic module or, alternatively, to an adjacent cell in a seconddirection within the photovoltaic module, forming a series configurationamong adjacent cells. The front or back current pathways may connect thephotovoltaic cell to an external terminal of the photovoltaic module inwhich it is located.

According to the first embodiment shown in FIG. 1, an electron reflectorlayer 130 is provided between the absorber layer 120 and the backelectrode layer 140. In this embodiment, the electron reflector layer130 is a zinc telluride (ZnTe) layer doped with copper telluride(Cu₂Te).

Doping is a process of intentionally introducing impurities into anotherwise extremely pure material, such as a semiconductor material, inorder to modulate its electrical or optical properties. In thisembodiment, the electron reflector layer 130 contains primarily ZnTewith a Cu₂Te dopant present in concentrations up to 5 at %. Thecompositional ratio (at %) of a compound, for example ZnTe:Cu₂Te isdetermined by comparing the number of Cu₂Te atoms in a given amount ofthe material with the total sum of ZnTe atoms and Cu₂Te atoms in thesame given amount. For example, where x=5 at %, there are 19 ZnTe atomfor every 1 Cu₂Te atom in a given amount of ZnTe:(Cu₂Te)_(5%) material.The electron reflector layer 130 may be between about 5 nm to about 25nm thick, or alternatively between about 15 nm to about 20 nm thick.

As an alternative configuration of the first embodiment, the electronreflector layer 130 may be deposited having a dopant gradient throughthe thickness of the layer. In such case the presence of the Cu₂Tedopant may vary so that there is less Cu₂Te present, for example about0.01 at %, where the electron reflector layer 130 is in contact with theabsorber layer 120 and more Cu₂Te present, for example about 5 at %,where the electron reflector layer 130 is in contact with the backelectrode layer 140 or the back contact layer 150. This gradient may bestepwise, where discrete adjacent layers of the material have differentamounts of dopant; or may be continuous so that the amount of dopantgradually changes through the thickness of the layer.

An electron reflector layer 130 according to the first embodiment may beformed through a variety of known deposition processes including, forexample, sputtering, physical vapor deposition, chemical vapordeposition, electro-chemical deposition, atomic layer deposition,thermal or electron-beam evaporation, and pulse laser deposition. Oneexemplary process for creating an electron reflector layer 130 wouldinclude the steps of: first, providing a substrate structure including abase layer, such as glass, with a TCO, window layer and absorber layerformed thereon within a deposition chamber. Second, evaporating amixture of ZnTe and Cu₂Te powders within the deposition chamber anddirecting the vapor across the substrate structure using an inertcarrier gas, such as a noble gas or nitrogen. Within the depositionchamber the substrate structure is at a temperature below thecondensation threshold of the material as the vapor is directed acrossthe substrate structure to facilitate the formation of the layer.

An alternative exemplary process for creating the electron reflectorlayer 130 may include a sputtering process, rather than an evaporationprocess. Sputtering is a process whereby atoms are ejected from a solidtarget material due to bombardment of the target by energetic particles,such as from an ion or plasma source. In the exemplary process, thesputtering target may be made of the mixture of ZnTe and Cu₂Te. Thematerial sputtered off the target is deposited onto the substrate in aninert gas ambient, such a noble gas, an argon gas, or other gas ambientincluding argon, hydrogen, nitrogen or combinations thereof. Thesputtering deposition may be processed in a reduced vacuum, betweenabout 0.01 mTorr to about 50 mTorr. The sputtering deposition may beprocessed at room temperature or at an elevated temperature betweenabout room temperature up to about 400° C.

The ratio of ZnTe to Cu₂Te powder in sputtering target or theevaporation source may about match the composition of the depositedlayer. For example, the sputtering target or evaporation source maycontain about 5% Cu₂Te powder with about 95% ZnTe powder to deposit amaterial layer of ZnTe doped to about 5% Cu₂Te. Alternatively, thecomposition of the powder mix in the sputtering target or evaporationsource may require a different composition to achieve the composition ofthe deposited layer. For example, the sputtering target or evaporationsource may contain a powder mix of about 7%, or up to about 10% Cu₂Tewith about 93% or about 90% ZnTe in order to achieve a deposited layerof ZnTe:(Cu₂Te)_(5%).

In order to form a gradient dopant concentration it may be necessary toevaporate the materials in multiple evaporation sources and mix them invapor form during the deposition process or by sputtering the materialsfrom targets having various compositions that could be co-sputtered atthe same time with a controlled deposition rate ratio to achieve thedesired gradient of the layer. Alternatively, the evaporation orsputtering process may be performed in sequential steps using multipleevaporations sources or multiple targets of various compositions.

Referring now to FIG. 2, a second embodiment is shown where like layersare designated as in the prior embodiment and described above. Aphotovoltaic device 200 according to the present embodiment includes awindow layer 115 deposited over a substrate structure including a baselayer 105 and a TCO layer 110. An absorber layer 120 is deposited overthe window layer 115. The window layer 115 and the absorber layer 120form a p-n junction in the photovoltaic device 200. An electronreflector layer 230 is deposited over the absorber layer 120. A backelectrode layer 140 and a back contact layer 150 are formed over theelectron reflector layer 230.

The electron reflector layer 230 according to the present embodiment isprovided between the absorber layer 120 and the back electrode layer140. In this embodiment, the electron reflector layer 230 is aZnTe/Cu₂Te alloy. The electron reflector layer 230 may be between about5 nm to about 25 nm thick or alternatively between about 15 nm to about20 nm thick.

Unlike the present embodiment, the doped ZnTe:Cu₂Te material of theprior embodiment maintains the crystal structure of a pure ZnTe materialwith the Cu₂Te present as impurities within the ZnTe crystal structure.According to the present embodiment, the electron reflector layer 230 isa ternary alloy of zinc, copper and tellurium having a formulation of(Cu₂)_(x)Zn_((1-x))Te, where x is between at least 5 at % and about 60at %. The ternary alloy forms a crystal structure of(Cu₂)_(x)Zn_((1-x))Te, with the presence of copper influencing themorphology, or form, of crystal structure within the material, unlike inthe first embodiment above.

As an alternative configuration of the second embodiment, the electronreflector layer 230 may be deposited having an alloy compositiongradient through the thickness of the layer. In such case the proportionof Cu₂Te in the alloy may vary so that there is less Cu₂Te present, forexample about 5 at %, where the electron reflector layer 230 is incontact with the absorber layer 120 and more Cu₂Te present, for exampleabout 25 at %, where the electron reflector layer 230 is in contact withthe back electrode layer 140 or the back contact layer 150. Thisgradient may be stepwise, where discrete adjacent layers of the materialhave different proportions of alloy components; or may be continuous sothat the alloy proportion gradually changes through the thickness of thelayer.

An electron reflector layer 230 according to the second embodiment maybe formed through a variety of known deposition processes including, forexample, sputtering, physical vapor deposition, chemical vapordeposition, electro-chemical deposition, atomic layer deposition,thermal or electron-beam evaporation, and pulse laser deposition. Oneexemplary process for creating the electron reflector layer 230 wouldinclude the steps of: first, providing a substrate structure including abase layer, such as glass, with a TCO, window layer and absorber layerformed thereon within a deposition chamber. Second, evaporating a powderof ZnTe/Cu₂Te alloy within the deposition chamber and directing thevapor across the substrate structure using an inert carrier gas, such asa noble gas or nitrogen. Within the deposition chamber the substratestructure is at a temperature below the condensation threshold of thematerial as the vapor is directed across the substrate structure tofacilitate the formation of the layer.

An alternative exemplary process for creating the electron reflectorlayer 230 may include a sputtering process, rather than an evaporationprocess. In the exemplary process, the sputtering target may be made ofan alloy of ZnTe and Cu₂Te. The material sputtered off the target isdeposited onto the substrate in an inert gas ambient, such a noble gas,an argon gas, or other gas ambient including argon, hydrogen, nitrogenor combinations thereof. The sputtering deposition may be processed in areduced vacuum, between about 0.01 mTorr to about 50 mTorr. Thesputtering deposition may be processed at room temperature or at anelevated temperature between about room temperature up to about 400° C.

The composition of the ZnTe/Cu₂Te alloy in the powder or the sputtertarget can match the composition of the deposited layer. For example,the sputtering target or evaporation source may contain a powder of(Cu₂)_(x)Zn_((1-x))Te where x is about equal to, for example, 15% todeposit a material layer of (Cu₂)_(x)Zn_((1-x))Te where x is equal toabout 15%. Alternatively, the composition of the powder alloy in theevaporation source may require a different composition to achieve thetarget composition of the deposited layer. For example, the evaporationsource may contain a powder mix of about 20%, or up to about 25% Cu₂Tewith about 80% or about 75% ZnTe in order to achieve a deposited layerof (Cu₂)_(x)Zn_((1-x))Te where x is equal to about 15%.

In order to form a gradient dopant concentration it may be necessary toevaporate the materials in multiple evaporation sources and mix them invapor form during the deposition process or by sputtering the materialsfrom targets having various compositions that could be co-sputtered atthe same time with a controlled deposition rate ratio to achieve thedesired gradient of the layer. Alternatively, the evaporation orsputtering process may be performed in sequential steps using multipleevaporations sources or multiple targets of various compositions.

Referring now to FIG. 3, a third embodiment of a photovoltaic device 300is shown where like layers are designated as in the prior embodimentsand described above. A photovoltaic device 300 according to the presentembodiment includes a window layer 115 deposited over a substratestructure including a base layer 105 and a TCO layer 110. An absorberlayer 120 is deposited over the window layer 115. The window layer 115and the absorber layer 120 form a p-n junction in the photovoltaicdevice 300. An electron reflector layer 330 is deposited over theabsorber layer 120. A back electrode layer 140 and a back contact layer150 are formed over the electron reflector layer 330.

The electron reflector layer 330 according to the present embodiment isprovided between the p-type absorber layer 120 and the back electrodelayer 140. In this embodiment, the electron reflector layer 330 includesmultiple sublayers 330 a and 330 b. The electron reflector sublayer 330a adjacent to the absorber layer 120 may be chosen to provide aneffective conduction-band energy bather while also maintaining anear-crystallographic compatibility with the absorber layer 120material. The electron reflector sublayer 330 b may be chosen to providea low resistance electrical connection with both the electron reflectorsublayer 330 a and the adjacent back electrode layer 140 or back contactlayer 150 material.

The sublayers 330 a and 330 b may be chosen from the group comprising:ZnTe:Cu₂Te as described in the first embodiment above, ZnTe/Cu₂Te alloyas described in the second embodiment above, ZnTe, Cu₂Te, ZnTe dopedwith elemental copper between about 0.001 at % to about 2 at %(ZnTe:Cu), and cadmium zinc telluride (CZT) having a composition ofCd_((1-x))Zn_(x)Te where x is between about 30% to about 70%. In oneexemplary configuration according to the third embodiment, the electronreflector sublayer 330 a may be ZnTe with an electron reflector sublayer330 b of ZnTe/Cu₂Te alloy. In an alternative exemplary configuration,the electron reflector sublayer 330 a may be CZT with an electronreflector sublayer 330 b of ZnTe:Cu₂Te. Other configurations are alsopossible. The total electron reflector layer 330 may be between about 5nm to about 50 nm thick.

The electron reflector layer 330 according to the third embodiment maybe formed through a variety of known deposition processes including, forexample, sputtering, physical vapor deposition, chemical vapordeposition, electro-chemical deposition, atomic layer deposition,thermal or electron-beam evaporation, and pulse laser deposition. Oneexemplary process for creating the electron reflector layer 330 wouldinclude similar steps as described above, first providing a substratestructure including a base layer, such as glass, with a TCO, windowlayer and absorber layer formed thereon within a deposition chamber.Then, sequentially evaporating a powder of material chosen for sublayer330 a within the deposition chamber and directing the vapor across thesubstrate structure using an inert carrier gas, such as a noble gas ornitrogen. That step is followed by a secondary deposition process withthe material chosen for sublayer 330 b. Within the deposition chamberthe substrate structure is at a temperature below the condensationthreshold of the material as the vapor is directed across the substratestructure to facilitate the formation of the layer. Alternativelysequential sputtering process as described above could be used todeposit the sublayers 330 a and 330 b together forming the electronreflector layer 330.

The window, absorber, and electron reflector layers described in theabove embodiments are crystalline solids that can be sequentially formedin thin films on a substrate structure that may include a base layer,TCO, and additional buffer layers, barrier layers or coatings. Inaccordance with the above disclosed embodiments, an electron reflectorlayer may be included in a photovoltaic device between the absorberlayer and the layer or layers forming a back current pathway.

A method of manufacturing a photovoltaic structure, as depicted in FIG.4, can include sequentially forming layers on a substrate. In a firststep 402, a TCO layer can be formed on a base layer, such as glass. In asecond step 404, a window layer can be deposited over the substratestructure including the previously applied TCO layer and base layer. Thewindow layer may include an n-type CdS semiconductor. In a third step406, an absorber layer can be deposited over the substrate structureincluding the window layer, TCO layer and base layer. The absorber layermay include a p-type CdTe semiconductor. In a fourth step 408, anelectron reflector layer may be deposited. The electron absorber mayinclude ZnTe:Cu₂Te, a ZnTe/Cu₂Te alloy, or a bi-layer structureincluding two materials chosen from the group comprising: ZnTe:Cu₂Te, aZnTe/Cu₂Te alloy, ZnTe, ZnTe:C, Cu₂Te, and CZT. In a fifth step 410, anactivation process as described below may be performed on the formedlayers. In a sixth step 412, a back electrode layer and a back contactlayer can be formed over the electron reflector layer.

A step of the method of manufacturing a photovoltaic structure mayinclude an expanded process as depicted in FIG. 5. The expanded processmay be substituted for any step requiring the formation of an alloyed ordoped material layer, regardless whether window layer, absorber layer orelectron reflector layer. In a first step 501 of the expanded process, afirst precursor layer, for example ZnTe, is deposited over a substratestructure. In a second step 503 of the expanded process, a secondprecursor layer, for example Cu₂Te, is deposited over the firstprecursor layer. In a third step 505 of the expanded process, thedeposited precursor layers are annealed to form desired final layerform, for example a ZnTe/Cu₂Te alloy layer.

As noted, in one embodiment of the expanded process, the first stepincludes depositing a ZnTe layer as the first precursor layer over asubstrate structure. The second step includes depositing a Cu₂Te layeras the second precursor layer over the ZnTe layer. The third stepincludes annealing the deposited precursor layers to form a ZnTe/Cu₂Telayer of the desired (Cu2)_(x)Zn_((1-x))Te compositional ratio. In analternative embodiment of the expanded process, the annealing of theprecursor layers occurs during the subsequent deposition or formation ofa back electrode or back contact layer.

Subsequent to formation of a layer, the structure may go through anactivation process. When a CdTe, CST, or other absorber layer is used,an activation step can include the introduction of a material containingchlorine to the semiconductor bi-layer, for example cadmium chloride(CdCl₂), as a bathing solution, spray, or vapor, and an associatedannealing of the absorber layer at an elevated temperature. For example,if CdCl₂ is used, the CdCl₂ can be applied over the absorber layer as anaqueous solution at a concentration of about 50-500 g/L. Alternatively,the absorber layer can be annealed with CdCl₂ by continuously flowingCdCl₂ vapor over the surface of the absorber layer during the annealingstep. Alternative chlorine-doping materials can also be used such asMnC₂, MgCl₂, NHCl2, ZnCl2, or TeCl2. A typical anneal can be performedat a temperature of about 200°-450° C. for a total duration of 90minutes or less, with a soaking time equal to or less than about 60minutes.

Inclusion of an electron reflector layer can impact the activationprocess during which certain atoms may migrate from the layers asdeposited to adjacent layers. For example, during the activation processthe copper present in the electron reflector layer may diffuse into theabsorber layer, such as CdTe as a dopant to the CdTe increasing thenumber of charge carriers (electrons) available to conduct as aphotogenerated current. A photovoltaic device as described in the firstembodiment above containing a ZnTe:Cu₂Te electron reflector layer 130may have an activation process including a thermal treatment from about250° C. to about 350° C., either with or without a chlorine-containingmaterial present. A photovoltaic device as described in the secondembodiment above containing a ZnTe/Cu₂Te alloy electron reflector layer230 may have a thermal treatment from about 200° C. to about 300° C.,either with or without a chlorine-containing material present. Aphotovoltaic device as described in the third embodiment abovecontaining a bi-layer electron reflector layer 330 may have a thermaltreatment from about 200° C. to about 350° C., either with or without achlorine-containing material present.

For each of the embodiments describing various photovoltaic devicesincorporating an electron reflector layer a multi-step activationprocesses or single activation steps may be used. With each desiredactivation mechanism such as semiconductor grain growth, chlorinediffusion, sulfur and selenium inter-diffusion into the layers,different thermal activation energy is required. Using a multi-stepprocess allows each activation mechanism to be optimized by selectingthe process temperature and duration.

As an example of a multi-step activation process, CdCl₂ can be appliedin a single step followed by annealing using a multi-step temperatureprofile. For example, the anneal temperature may be ramped up to about425° C. first, held there for a period of time (e.g. 1-10 minutes) andthen ramped up further to 450°-460° C. and held there for an additionalperiod of time (e.g., 1-10 minutes) before ramping the annealtemperature back down. This temperature profile for the above annealresults in different crystallinity characteristics of the CdTe materialthan those of a device activated in a single anneal step at 425° C. oralternatively at 450°-460° C. As an extension or alternative to thisapproach, multiple CdCl₂ applications, each paired with annealing atvaried times and temperatures may also be used to achieve desired layercharacteristics.

From the foregoing description, one ordinarily skilled in the art caneasily ascertain the essential characteristics of the embodiments and,without departing from the spirit and scope thereof, can make variouschanges and modifications to adapt the disclosed embodiments to varioususages and conditions.

What is claimed:
 1. A photovoltaic structure comprising: a substratestructure including a base layer, a transparent conductive oxide layer,at least one semiconductor layer; and an electron reflector layerprovided over the substrate structure.
 2. The photovoltaic device ofclaim 1, wherein the electron reflector layer comprises zinc telluridedoped with copper telluride.
 3. The photovoltaic device of claim 2,wherein the zinc telluride is doped with copper telluride at aconcentration between 0.01% and 5%.
 4. The photovoltaic device of claim2, wherein the zinc telluride is doped with copper telluride at aconcentration between 2% and 5%.
 5. The photovoltaic device of claim 1,wherein the electron reflector layer comprises zinc telluride alloyedwith copper telluride to form (Cu₂)_(x)Zn_((1-x))Te.
 6. The photovoltaicdevice of claim 5, wherein the electron reflector layer comprises(Cu₂)_(x)Zn_((1-x))Te, where x is between 5% and 60%.
 7. Thephotovoltaic device of claim 5, wherein the electron reflector layercomprises (Cu₂)_(x)Zn_((1-x))Te, where x is between 20% and 35%.
 8. Thephotovoltaic device of claim 1, wherein the electron reflector layercomprises a bilayer of two sublayers wherein a first sublayer materialis chosen from: zinc telluride doped with copper telluride, and zinctelluride-copper telluride alloy; and where the second sublayer materialis chosen from: zinc telluride doped with elemental copper, zinctelluride, cadmium zinc telluride, and copper telluride.
 9. Thephotovoltaic device of claim 8, wherein the first sublayer is zinctelluride doped with copper telluride at a concentration of between0.01% and 5%; and where the second sublayer is zinc telluride.
 10. Thephotovoltaic device of claim 8, wherein the first sublayer is zinctelluride alloyed with copper telluride at a concentration of between 5%and 60%; and where the second sublayer is cadmium zinc telluride.
 11. Aprocess for manufacturing a photovoltaic device comprising the steps of:forming an electron reflector layer over a substrate structure includinga base layer, a transparent conductive oxide layer, and at least onesemiconductor layer; and activating the photovoltaic device by applyinga thermal treatment.
 12. A process for manufacturing a photovoltaicdevice as in claim 11, wherein the step of forming an electron reflectorlayer is performed by depositing a zinc telluride doped with coppertelluride at a concentration of between 0.01% and 5% by a vapordeposition process.
 13. A process for manufacturing a photovoltaicdevice as in claim 11, wherein the step of forming an electron reflectorlayer is performed by depositing a zinc telluride doped with coppertelluride at a concentration of between 0.01% and 5% by a sputteringprocess.
 14. A process for manufacturing a photovoltaic device as inclaim 11, wherein the step of forming an electron reflector layer isperformed by depositing a zinc telluride alloyed with copper tellurideat a concentration of between 5% and 60% by a vapor deposition process.15. A process for manufacturing a photovoltaic device as in claim 11,wherein the step of forming an electron reflector layer is performed bydepositing a zinc telluride alloyed with copper telluride at aconcentration of between 5% and 60% by a sputtering process.
 16. Aprocess for manufacturing a photovoltaic device as in claim 11, whereinthe step of forming an electron reflector layer is performed bysequentially forming a first sublayer and a second sublayer.
 17. Aprocess for manufacturing a photovoltaic device as in claim 16, whereinforming at least one of the first sublayer and the second sublayer isperformed by depositing a zinc telluride doped with copper telluride ata concentration of between 0.01% and 5% by a vapor deposition process.18. A process for manufacturing a photovoltaic device as in claim 16,wherein forming at least one of the first sublayer and the secondsublayer is performed by depositing a zinc telluride doped with coppertelluride at a concentration of between 0.01% and 5% by a sputteringprocess.
 19. A process for manufacturing a photovoltaic device as inclaim 16, wherein forming at least one of the first sublayer and thesecond sublayer is performed by depositing a zinc telluride alloyed withcopper telluride at a concentration of between 5% and 60% by a vapordeposition process.
 20. A process for manufacturing a photovoltaicdevice as in claim 16, wherein forming at least one of the firstsublayer and the second sublayer is performed by depositing a zinctelluride alloyed with copper telluride at a concentration of between 5%and 60% by a sputtering process.